Precision of Math Node?
For values Blender uses IEEE single precision floats. This gives a precision of about 7 decimal digits. The displayed value is rounded to 3 decimal places and the displayed value when editing the field to 6 decimal places (probably because the precision limitations).

The RMB value viewer in image editor of render output is rounded to 5 decimal places, but render values will render in full 32bit. Stored and rounded in format you choose.

It is also unlikely you are experiencing limitations in floating point representation:

$1/119617 = 0.000008360015716829547 $

When rendering this and storing with 32bit .EXR you get:



A sampled value of 0.00001 in Image Editor (5 decimal rounding), a value of 0.000008 in Data-Blocks outliner view (6 decimal rounding):



And value of 0.000008360016181541141 that is saved in the file by inspecting it through the console:



That's an error of 0.00000000000046471159

All of these cannot explain the 0.0001 error you are seeing. The problem is somewhere else than in precision of Blender math or your math is more complicated than what it seems and your floating point limitations snow-balled into the 0.0001 error.